This invention relates to a method for reducing the P.sub.2 O.sub.5 content of aqueous aluminum fluoride (AlF.sub.3) enriched crystallization mother liquors such as those encountered in the production of AlF.sub.3 by the reaction of an aluminum-containing material and P.sub.2 O.sub.5 contaminated fluorine containing acid, for example, the fluorine acids commonly produced as by-products of wet process phosphoric acid manufacture.
AlF.sub.3 is produced by reacting an aluminum source such as high purity aluminum, alumina, bauxite or clay with a fluorine-containing inorganic acid such as hydrofluoric acid (HF), fluosilicic acid (H.sub.2 SiF.sub.6), or a mixture thereof. The AlF.sub.3 product is recovered by subjecting the liquid phase of the reaction mixture, in which the AlF.sub.3 is dissolved, to crystallization in order to produce crystals of hydrated AlF.sub.3 which can then be dried and dehydrated to yield the desired AlF.sub.3. Examples of various processes for producing AlF.sub.3 are described in commonly assigned co-pending U.S. application Ser. No. 496,577 filed Aug. 12, 1974 in the names of T. K. Wiewiorowski and D. J. Miller entitled "Production of Alumina From Ores", U.S. Pats. 508,796, 1,391,172, 1,403,183, 2,842,426 and 2,920,938 and British Patents 15,083 and 643,379, said patent application and patents incorporated herein by reference.
One plentiful and relatively inexpensive source of the fluorine acid required in such processes is by-product acid from the manufacture of wet process phosphoric acid. As is known, the wet process generates large amounts of acids such as HF and H.sub.2 SiF.sub.6, as well as mixtures of SiF.sub.4 and HF vapors which can be absorbed in aqueous streams to form H.sub.2 SiF.sub.6. Unfortunately, the by-product fluorine acids from wet process phosphoric acid manufacture usually contain small concentrations of P.sub.2 O.sub.5 which, if not removed, eventually build to the point where they create intolerable impurity levels in the AlF.sub.3 product and in process recycle streams enriched in AlF.sub.3. One such recycle stream is the mother liquor produced in the above described crystallization of the hydrated AlF.sub.3 from the liquid phase of the reaction mixture, and it is toward the removal of P.sub.2 O.sub.5 contaminants from this recycled mother liquor that the present invention is directed.
The crystallization of hydrated AlF.sub.3 is rarely, if ever, quantitative. The normal experience is that this crystallization is only about 70 to 90% efficient. Consequently, substantial amounts, e.g., where to 30% of the AlF.sub.3 reaction product, do not crystallize, and remain dissolved in the crystallization mother liquor. For economic reasons, it is considered necessary to recycle this AlF.sub.3 enriched mother liquor to the reaction step or any other convenient step of the process in order to recover its substantial and valuable AlF.sub.3 content. However, much of the P.sub.2 O.sub.5 introduced to the process by the P.sub.2 O.sub.5 contaminated fluorine acid remains in this mother liquor and often prevents the recycle of the mother liquor because the P.sub.2 O.sub.5 then shows up, in excessive amounts, as an impurity in the AlF.sub.3 product. The major use of aluminum fluoride is in the electrolytic reduction of alumina to aluminum metal, and stringest specifications are imposed for the upper limit of P.sub.2 O.sub.5 in AlF.sub.3 intended for this purpose. The current commercial specification for such AlF.sub.3 permits a maximum of about 0.02% by weight P.sub.2 O.sub.5, a level low enough to often prohibit recycle of the P.sub.2 O.sub.5 contaminated crystallization mother liquor. It is evident therefore that the overall efficiency of the AlF.sub.3 manufacturing process could be improved if the P.sub.2 O.sub.5 level in the crystallization mother liquor could be reduced to the point were the mother liquor could be recycled to recover its substantial AlF.sub.3 content without the risk of unduly contaminating the AlF.sub.3 product with P.sub.2 O.sub.5.
It is, therefore, a general object of this invention to provide an expeditious and economically attractive method for reducing the P.sub.2 O.sub.5 levels in the aqueous AlF.sub.3 enriched crystallization mother liquors generated in the production of AlF.sub.3 to the point where the mother liquor can be recycled to the process without causing undue P.sub.2 O.sub.5 contamination of the AlF.sub.3 product.
It is another object of the invention to provide a method for increasing the recovery of high purity AlF.sub.3 in an AlF.sub.3 manufacturing process which generates AlF.sub.3 enriched mother liquors which are contaminated with P.sub.2 O.sub.5 values.
It is another object of the invention to provide a method which renders the use of P.sub.2 O.sub.5 contaminated fluorine acid, such as that produced as a by-product in wet process phosphoric acid manufacture, a more attractive source of fluorine for the preparation of AlF.sub.3.
These and other objects of the invention will be apparent to those skilled in the art upon consideration of this specification in its entirety including the accompanying drawing illustrating a preferred embodiment of the invention.